What are the most popular POS systems used in US bars and pubs?

For bar point of sale a couple things you'll want to make sure your pos does the following:

• pre-authorization, duplicate charge notification
• blind closing
• ID scanners
• Dual inventory control if you have two bar tending stations, so bartenders don't have to cross over each other.
• Happy hour pricing

It wouldn't make sense to invest in an expensive system if it didn't have all the features you're looking for especially for your industry. Liquor inventory management requires another level of understanding so if the company you choose doesn't specifically cater or has a history with bar pos then you may run into limitations into what you want it to manage.  
Check out Saitec POS Solution is a retail POS system. It offers POS Solution which is easy to use and learn for employees with basic functionalities. This custom POS system is used for each small business like retail stores, bars and pubs.